The present invention relates to area-efficient reconstruction filters, particularly for current-driven digital-to-analog converters (DAC).
Digital-to-analog converters are conventionally used very frequently in integrated circuits.
Since these converters are sampled-data circuits, in addition to generating the intended analog signal in the correct frequency range or base band, they also produce in output an undesirable duplicate image of the signal, generally designated as xe2x80x9cimagingxe2x80x9d, as shown in FIG. 1, which plots the output of the DAC as a function of the frequency f.
The chart shows that in addition to the output signal, designated by S (where B is the base band), there is also a duplicate image of the signal S which is centered around the sampling frequency fS of the DAC.
In order to eliminate this duplicate image, a continuous-time low-pass reconstruction filter is usually introduced and placed downstream of the DAC, as shown in FIG. 2.
In this Figure, the reference numeral 1 designates an N-bit DAC, where b0, b1, . . . , baxe2x88x921 are the input bits of the DAC and VDAC and IDAC are, respectively, the output voltage and the output current of the DAC. The reference numeral 2 instead designates a continuous-time low-pass reconstruction filter arranged downstream of the DAC 1 and V0 is the output voltage.
The reconstruction filter 2 must provide high attenuation for frequencies close to the sampling frequency fS of the DAC, but at the same time it must be efficient in terms of area occupation if the DAC is to be used in an integrated circuit, where of course the requirement of minimum area occupation is one of the most important factors.
It is known to those skilled in the art that these are two mutually contrasting requirements.
It is therefore necessary to achieve a compromise, shown in FIGS. 3a and 3b. The filters shown in these figures are second-order low-pass filters. The solution shown in FIG. 3b, however, is the one that is practically mandatory when working with supply voltages of less than 3V
The solution of FIG. 3b is rather area efficient when the input signal is a voltage, but it would be highly insufficient when the input signal must be a current, as shown in FIG. 3c by applying only the Norton equivalent to the input of FIG. 3b. 
A numeric example is now described to clarity the above explanation.
Assume that a DAC has been devised which has a full-scale voltage output ViPS=0.5 V and that a full-scale voltage VGPS=0.5 V from the reconstruction filter is required as output. Assume also that a cutoff frequency of approximately 270 kHz is chosen for the filter.
The values of the components of FIG. 3b will be as follows:
R1=R2=R3=50 kohm C1=25.2 pF C2=5.6 pF
Assume also that one intends to use a DAC with a full-scale current output IDACFS=160 xcexcA and that one seeks a full-scale output voltage VCFS=0.5 V from the reconstruction filter with a frequency response that is identical to that of the filter used previously with the voltage-output DAC.
Since one must have R3IDACFS=VOFS, then R3=3.125 kohm and therefore R1=R2=R3=3.125 kohm.
Therefore, in order to have the same frequency response as the preceding filter, the values of C1 and C2 must be 403.2 pF and 89.6 pF respectively.
Accordingly, the area occupied on the silicon in order to integrate the values of these components is approximately sixteen times greater than the area occupied to integrate the components of FIG. 3b, due to the relatively low specific capacitance that can be provided in integrated circuits.
Therefore, the reconstruction filter, in the case of a current input (and therefore of a current output of the DAC), is very wasteful from the point of view of the area occupied on the silicon wafer.
An aim of the present invention is to provide an area-efficient reconstruction filter, particularly for current-driven digital-to-analog converters.
Within the scope of this aim, an object of the present invention is to provide a reconstruction filter for current-driven digital-to-analog converters which is optimized in view of its integration in an integrated circuit.
Another object of the present invention is to provide a reconstruction filter for current-driven digital-to-analog converters in which the reconstruction filter with current input has an area occupation at least as good as the corresponding reconstruction filter with voltage input.
Another object of the present invention is to provide a reconstruction filter with current input having the same transfer function as a similar reconstruction filter with voltage input.
Another object of the present invention is to provide a reconstruction filter that is highly reliable, relatively easy to produce, and cost-competitive.
This aim, these objects and others which will become apparent hereinafter are achieved in one embodiment by an area-efficient reconstruction filter, particularly for current-driven digital-to-analog converters, including: an input node for receiving the input current signal; an operational amplifier having first and second inputs and an output at which the output voltage signal is produced; a first resistor coupled between the output of the operational amplifier and the input node; a second resistor connected to the first input of the operational amplifier; and a third resistor connected between the input node and the second resistor. The reconstruction filter may also include a fourth resistor coupled between the input node and a reference voltage.